Method of recovering chromates and vanadates from aqueous solution



. line roasting of chromite ores.

Patented Feb. 10, 1953 METHOD OF RECOVERING CHROMATES AND VANADATES FROM AQUEOUS SO- LUTION Tom S. Perrin and Robert G. Banner, Painesville, Chic, assignors to Diamond Alkali Company, Cleveland, Ohio, a corporation of Delaware No Drawing. Application April 28, 1950, Serial No. 158,918

6 Claims.

This invention relates to a method for the recovery of vanadates from substances containing lead vanadate, and more particularly relates to an improved method for recovering vanadates from precipitates containing lead vanadate and lead chromate.

It has been proposed to decompose substances containing lead vanadate with sulfuric acid to recover the vanadium values therefrom as a solution of soluble vanadic acid and to oxidize the solution thus obtained to precipitate the vanadium values from the solution as a salt of hexametavanadic acid. More specifically, in application Ser. No. 59,935, filed November 13, 19%, in the name of Tom S. Perrin and another (now Patent No, 2,583,591, dated January 29, 1952), a method is taught for the purification of chromate solutions, with respect to vanadate impurities therein, such as process liquors obtained in the commercial production of chromates by the alka- The method there described comprises combining a lead salt with a solution containing a preponderance of alkali metal chromates and a minor proportion of alkali metal vanadates to co-precipitate substantially all of the vanadates together with a minor proportion of the chromates contained therein as lead chromate and lead vanadate, and the recovery of the vanadium values from the coprecipitate. The co-precipitate of lead chromate and lead vanadate obtained by this method consists essentially of substantially all of the vanadium in the process liquor as lead vanadate, a minor proportion of the chromium contained therein as lead-chromate, and possibly some exfrom the mother liquor by filtration and the filtrate obtained is returned to the main stream of the commercial process in order to recover the remaining chromate values therein. The precipitation step results in a reduction of vanadate concentration in the process liquor to within a tolerable limit in terms of the uses to which the chromates and bichromates obtained therefrom are subsequently to be put.

As disclosed in the above-noted application, a precipitate containing the bulk of the vanadium removed from the process liquor as lead vanadate, may be treated with dilute sulfuric acid, whereby the vanadate and chromate values of the precipitate are converted to soluble acids and the lead values are converted to insoluble lead sulfate. The insoluble lead sulfate is separated from the liquor containing the soluble Vanadic and chromic acids by filtration and the vanadium values contained in the filtrate are treated with an oxidizing agent in order to insure that the vanadium values inusolution are at their highest state of oxidation prior to further treatment of the solution to precipitate the desired salt of hexametavanadic acid.

The need for the oxidizing agent at this point in the process arises from the fact that the use of sulfuric acid in concentrations above-about 45% and at the temperatures normally employed in this and other commercial vanadium leaching processes, results in the chemical reduction of some of the vanadates contained in the acid leach solution to an extent that subsequent p'recipitation of the vanadium as a di-sodium or diammonium salt of hexametavanadic acid is inefiective to recover the vanadium values contained in the solution in any but commercially uneconomical amounts.

The present invention is directed broadly to an efficient method for the recovery of vanadium from substances containing lead vanadate by leaching such substances with dilute sulfuric acid without the necessity of resorting to an oxidizing agent to insure that the vanadium values contained in the solution are in their highest state oi oxidation. Moreover, the present invention represents an improvement in the method described in application Ser. No. 59,935 (Patent No. 2,583,591), referred to hereinabove, in that it is primarily directed to the recovery of vanadates from the co-precipitate of lead chromate and lead vanadate obtained by that method. Thus, in the method of recovering vanadates from substances containing lead vanadate by the decomposition of such substances with sulfuric acid and recovering the vanadium values thereof as a salt of hexametavanadic acid, the present invention is directed to the improvement which consists essentially in leaching such substances 'with sulfuric acid of a concentration substantially within the range of 3472 %-42 /2% by weight and at a temperature substantially within the range of 25-70 C., separating the solution comprising vanadic acid irom the lead sulfate thus formed, adjusting the pH of the solution to a point Within the range of 1-3 with a base selected from the group of sodium and ammonium bases, and heating the solution to a temperature within the range of C. to the boiling point of the solution to precipitate a salt of hexametavanadic acid.

The material containing lead vanadate to be treated in accordance with this invention may be in any suitable form, such as a finely divided precipitate of lead vanadate, or a finely ground mineral, such as ground vanadinite, in order to promote intimate contact of the leaching sulfuric acid With the vanadate containing material. For convenience the method will be described in conjunction with the co-precipitate of lead vanadatelead chromate obtained by the method of application Ser. No. 59,935 (Patent No. 2,583,591), referred to hereinabove. V

Finely divided material containing lead vanadate precipitate, lead chromate, and optionally other impurities is slurried with a dilute sulfuric acid having a concentration substantially within the range of 35 /2%-42 /z% by .weight, which is equivalent to the dilution of one volume of commercial 95% sulfuric acid with 2 /23 /2 volumes of water. Ideally, the sulfuric acid used to leach the lead vanadate containing materialhas a concentration of the order of 38 which is equivalent to the dilution of one volume of commercial 95% sulfuric acid with three volumes of water. In order to obtain the maximum ultimate yield of vanadate as a salt of hexametavanadic acid, it is preferable to avoid concentrations of -sulfuric acid above about: 43% by weight since such concentrations of acid may affect the chemical reduction of the leach vanadic acid as noted hereinabove, whereby the ultimate yield of vanadate precipitate is materially le sened.

. In addition to the concentration of the sulfuric acid used to leach the precipitate, the temperature at which leaching is carried out is important. There is basis for a hypothesis that leaching at temperatures above about l" C. allows coagulation of a salt of hexametavanadic acid which remains behind with the lead sulfate precipitate also obtained during the leaching step. The slurry of the dilute sulfuric acid and lead vanadate containing material is agitatedfor a period from 5 to 15 minutes, preferably about minutes, and at a temperature substantially within the range of 2570 C., the lower temperatures within this range corresponding to the longer contact time within the intervals given and the higher temperature range corresponding to the shorter contact time within these intervals. After thedigestion of the lead-vanadate containing material in the sulfuric acid for the time and at the temperatures prescribed above, the mixture is filtered in order to separate the solution from theprecipitate of lead sulfate thus obtained. It has been found that filter media, such as fritted glass, ceramic filters, filter cloth-made of synthetic resinous polymers, such as vinyl chloride-acrylonitrile, polyethylene, vinylidene chloride, and a commercial filter cloth known in the-trade as Vinyon N #702, comprising vinyl chloride-vinyl acetate co-polymer, are suitable in thepresent process.

. After filtration ofthe supernatant liquid from the reaction mass, the precipitate is washed on "the filter with water, preferably water substantially completely free of iron, since ferric ions present in the filtrate a this point in the method of the present invention result in the precipitation of iron vanadate, which will adversely affeet the recovery of the vanadate values contained in the solution ultimately as a salt of hexametavanadic acid.

If the vanadate containing filtrate at this point in' the process contains substantially less than 18-20 gms. of vanadate calculated as NaVOx per liter of solution, the vanadate concentration is adjusted to approximately this minimum by concentratingthe solution, adding sodium vanadate thereto, or by diluting the solution if necessary to a concentration somewhere near, but not below, this minimum concentration. The solution is then heated to a temperature within the range of approximately 80 C. to the boiling point of the solution and the pH of the solution adjusted to a point within the range of l-3, preferably,

however, within the range of 2-2.5, by'means of a base of the class of sodium and ammonium bases,

. of chromates from chromite ores, it is advantageous both to the recovery of vanadium and the purification of the chromate liquors with respect to vanadium, to employ the excess alkali contained in the alkaline leach liquors from the leaching of the alkaline roast mass to adjust the pH of the vanadic acid solution to the desired point.

After adjusting the pH, the solution containing the vanadate values leached from the lead vanadate containing substance is agitated and heated for a period of approximately 3-hours, during which the vanadate valuesare precipitated from the solution as a di-sodium or di-ammonium salt of hexametavanadic acid. The precipitate is separated from the mother liquor by simple filtration, washed several times on the filter in order to remove the grossly held mother liquor, and subsequently treated by known methods in order to recover the vanadium values thereof as substantially pure vanadium pentoxide (V205).

It has also been found that where the improved method of the present invention is used in connection with a commercial chromate recovery process employing the alkaline roasting of a chromite ore and the subsequent leaching of the alkali roast, a cyclic method for the recovery of both the chromate values and the vanadate values of the chromite ore may be effected, which includes adjusting the pH of the sulfuric acid leach liquors from the leaching of the co-precipitate of lead chromate-lead vanadate, as noted hereina'bove, with the alkaline leach liquor from the leaching of the alkali roast material, and recyclin the filtrate from the separation of the salt of hexametavanadic acid ultimately obtained to the main stream of the chromate recovery of such process.

When the present invention is thus integrated with such a chromate recovery process, it has been found that there is, over an extended period of time, a very gradual increase in the vanadium content of the co-precipitate of lead chromate lead vanadate, and that it is preferable therefore in so integrating the method of the presentinvention periodically to leach the lead sulfate filter cake with additional amounts of dilute sulfuric acid (1 volume of commercial sulfuric acid diluted with 2.5-3.5 volumes of water) and to return the leach liquor thus obtained'to a point in the commercial chromate recovery process upstream from the point where lead sulfate is added to the chromate liquors to co-precipitate lead chromate and lead vanadate. However, when thus employing the method of the present invention, there is no increase in the vanadate concentration of the chromate liquors subsequent to the point of treating such liquors with lead sulfate in order to remove the vanadate therefrom, and thus a substantially constant but very low vanadium concentration is maintained in the chromate liquors, whereby the chromate and bichromate salts obtained in the chromate recovery process contain a sufficiently small amount of vanadium to meet the purity specifications required by leather tanners and chromate pigment manufacturers.

' In order that those skilled in the art may better understand the method of the present inven- I contained in the filter cake.

tion and by what manner the same'may be carried into effect, the following specific examples are offered:

Example I A chromite ore is subjected to an alkaline roast with soda ash at a temperature of the order of 1095 C. (2000 F); the roast is leached with water and the pH of the alkaline leach liquor adjusted to a point within the range of approximately 8.5-9 in order to precipitate alumina hydrate therefrom. Thereafter, the neutralized liquor is filtered in order to separate the alumina precipitate therefrom and the filtrate taken for the purposes of this example is found to contain 374.4 gms. per liter of NazCrzOrZHzO and 1.46 gms. NaVOg per liter. 86 liters of such liquor are slurried with a mass of basic lead sulfate (PbSO4-PbO) weighing 2.99 kilograms, and combined with distilled water to form a paste at a temperature of 80 C. The mixture of chromate liquor and basic lead sulfate is agitated at a temperature within the range of 80-90 C. for a period of approximately minutes, and the precipitate of lead chromate-lead vanadate thus formed separated from the chromate containing liquor by filtration, and washed with water on the filter. Analysis of the filtrate thus obtained shows that 94% of the vanadium values originally contained in the chromate liquor have been A portion of this precipitate containing 23.4

gms. of vanadate calculated as NaVOs is leached with an amount of sulfuric acid chemically equivalent to 1.2 times theequivalents of lead The acid is used in the form of a dilute solution (48%) obtained by diluting 1 volume of commercial 95% sulfuric acid with 2 volumes of distilled water, and the mixture of dilute sulfuric acid and filter cake stirred for a period of 5-10 minutes at 95 C. The solution and precipitate are separated by filtration on a fritted glass filter and the precipitate washed twice with water. The solution is combined with 410 mls. of vanadate and chromate-containing alkaline leach liquor obtained as described in the forepart of this example, (prior to the precipitation of the alumina from the leach liquor), to adjust the pH of the acid filtrate to 2.0. The total volume of the solution at this time is 1870 mls. and the total vanadate content of the solution calculated as NaVOais found to be 24.0 gms. (The increase in the amount of vanadate is due to the vanadate contained in the alkaline leach liquor added to adjust the pH.) The resulting liquor is heated at a temperature of 90 C. and agitated for a period of 3 hours, after which the liquor and the precipitate of the di-sodium salt of hexametavanadic acid, formed during the agitation period, are allowed to stand over-night. 53% of the vanadate calculated as NaVO-a and originally contained in the co-precipitate of lead chromate-lead vanadate is thus recovered as the di-sodium salt of heximetavanadic acid.

Example II A second portion of the precipitate of lead chromate-lead vanadate as obtained in Example I and containing the same amount of vanadate calculated as NaVOa is leached with a solution of sulfuric acid obtained by diluting one volume of commercial 95% sulfuric acid with 3 volumes of water (38%), and containing an amount .of sulfuric acid chemically equivalent to'1;2 times the amount of lead contained in the precipitate. The mixture of sulfuric acid and precipitate is heated to a temperature of 95 C. and agitated for a period of 10 minutes, after which the precipitate formed during the reaction is separated from the mother liquor by filtration through a fritted glass filter, washed with 250 mls. of water, then 150 mls. of water, to give a filtrate having a total volume of 1040 mls. Analysis of the filtrate shows that the vanadate content calculated as NaVOs is 21.1 gms. of that contained in the portion of the co-precipitate prior to leaching). The filtrate is divided into two equal portions. The first portion is combined with 8.84 gms. of lead peroxide and the mixture heated to C. with agitation for a period of 5 minutes. Thereafter, the mixture is filtered, washed with water, the pH of the filtrate adjusted to 2 (with 190 mls. of chromate and vanadate-containing alkaline leach liquor as described in the forepart of Example I), and the filtrate agitated for a period of 3 hours at 90 C., at the end of which time 94.8% of the vanadium (corrected for the vanadium content of the leach liquor) contained in the filtrate from the leaching of the chromate lead vanadate cake is recovered.

The second portion of the above filtrate is treated with 190 mls. of the leached liquor as described in the forepart of Example I (prior to the removal of alumina therefrom) and agitated for a period of 3 hours at 90 C., at the end. of which period the di-sodium salt of hexametavanadic acid which precipitates from the solution represents 85.4% of the vanadate calculated as NaVOs present in the solution.

Ezrample III A further portion of the lead chromate-lead vanadate cake as obtained in Example I is treated with 492 mls. of 38% sulfuric acid and the mixture thus obtained heated to 70 C. and agitated for a period of 10 minutes, after which the mixture is filtered and the filter cake thus obtained washed with two 150 mls. portions of water, the filtrate and washings being combined and diluted to give a total volume of the filtrate of 1240 mls. Analysis of this filtrate shows that 10.0% of the vanadate content of the lead chromatelead vanadate cake has been leached out by this treatment. The pH of the filtrate is a'diustedfto 2 with aqueous sodium hydroxide, the solution heated to 90 C. and agitated for a period of 3 hours, after which the precipitate is .allowedqito settle and is separated from the supernatant liquid by filtration, washed'with water, and dried. The dried precipitate showed that 86.2% of the vanadium values leached from the co-precipitate of lead chromate-lead vanadate arerecovered as the (ii-sodium salt of hexametavanadic acid.

Example IV 25 liters of a chromate and vanadate-containing liquor obtained as in Example I are placed in a -liter tank and. heated to a temperature of 80 C. 870 gms. of basic lead sulfate (PbSOe-PbO) are slurried with water and added to the chromate liquor with agitation. The precipitate of lead chromate-lead vanadatethus obtained is separated from the solution by filtration and the filter cake washed with waterto obtain a filtrate volume of 31 liters, containing 3.2 gms. of vanadate calculated asNaVOa: This represents a removal of approximately 91.5% of tofiltration through commercial #702? filter .cloth,1and vthefilter cake washed with..180..mls.:of waten. Analysis of the filtrate shows that 95 ofthevanadate contained in the co precipitate cake has been leached out during i the above process.

:justedto'2 with aqueous sodium: hydroxide, agitated-fora period of 3 hours at 90 C. to coagu- I r late 'thfiidi'eSOdlllfllSSdlt of hexametavana'dici acid, iafterwhichthe precipitate is separated from. the mother liquor by 1"1ltration,'.washed, dried, and weighed. .The' weight ofxtheprecipitate shows i that-95.2%. of thetyanadate-yalues contained in I. the filtrate from the-leaching of the co-precipitate cakeis recovered rasqsthef di-sodium .salt of .hexametavanadic'acid. i

1 While there have been. described .various' em bodimentsv of; the invention, the 1 methods dezscribed .are .not intended. to. be: understood as H limitingthe scope of the invention as it isrealized that changes.therewithin arepossible and it is further intended that: each element-.recited' .Nana'date contained inithe'originalsolution; The

.ll ilter'cake iSifOllIl'd to weigh 1370 gms'. when-wet and is divided-"into: equaliparts containing..3.26 l gms. of vanadate :calculated asINaVOa.

.Oneportion of the filter .cake containing 8.26

in thefilter cake have been leached by the above process. Thereafter, the pH in the filtrate. solutio'n is adjusted to -2- with aqueous sodium hydioxide, heated to'90' C. and agitated for a period of '3 hours. heating and agitation is separated from the solu- "tion byfiltration, washed, and it is found that 85% ofthe vanadate values leached from the pre- "cipitate cake is recovered as a di-sodium salt of The precipitate formed during the hexametavanadicacid.

.. Example V "Anotherportion of the co-precipitate cake ob- .1 tained in' Example IV and'containing the same amount of vanadate calculated as NaVOz is leached with 180 mls. of dilute sufuric acid'obtained by -diluting' i one volume of commercial 95% sulfuric acid with -3 volumes of waterand at a temperature of 60 C. The mixture of sulfuric acid and c-o-precipitate cake is agitated for a period of 10 minutes; the mixture is subjected The pH of the filtrate is adin any of the following claims is to be understood as referring to all equivalent elements for accomplishing.;substantially..;the same :results in substantially the same or equivalent manner, it being intended to cover the invention broadly in" 1 whatever form its principle maybe utilized.

What is claimed is: 1 1;. In themethod of recovering vanadates-from a:precipitate containing leadvanadateby. de-..

- composing said precipitate with sulfuric acid and recovering the vanadiumvalues of such precipitate as a salt of hexametavanadic acid, theimprovementwhich consists essentially in leaching Vinyon N said precipitate with sulfuric acid of a concentra- 'tion substantially. .within the range of. 34.5%-

42.5% and at a temperature substantially within the range of 25-70 .C., to convert said lead vanadate to vanadic acid and precipitate lead sulfate, separating the solution of vanadicacid from the lead sulfate, adjusting the pH ofthe solution t a point within the range of 13 with a base selected from the group consisting of sodium and ammonium bases, and heating the solution to a temperature within the range of 80 C. to the boiling point of the solution to precipitate a salt of hexametavanadic acid.

2(The method of claim 1 wherein said pre- 1 cipitate containing lead vanadate is a co-precipious solution comprising a preponderance of alkali metal vanadates, which includes adding a soluble lead compound which provides a source of lead ions in such solution to co-precipitate substantially all of the vanadates and a minor proportion of the chromates therein as their lead salts,

separating the co-precipitate from the body of the solution of alkali metal chromates, leaching said ce-precipitate with sulfuric acid of a concentration substantially within the range of 34.5 42.5% and at a temperature substantially within the range of 25-'? C., separating the solution of vanadic and chromic acids from the lead sulfate thus formed, adjusting the pH of the solution t a point within the rang-e of 1-3 with a base selectedfrom the group consisting of sodium and ammonium bases, heating the solution to precipitate a salt of hexametavanadic acid, and separating the precipitate from the chromic acid solution.

4. The method of claim 3 in which the lead sulfate is recycled to further quantities of alkali metal chromate solution containing alkali metal vanadates, and in which the chromic acid solution 1 obtained in the final step is recycled to said alkali metal chromate solution upstream from the point at which said lead sulfate is recycled thereto.

, 5.,.Th method of claim 4 wherein said lead sulfate isperiodicallyleached with sulfuric acid ..of a concentration within the range of 34.5%-

.'..42.5%. prior to the recycling of saidlead sulfate to said alkali metal chromate solution, and where- ..,in..the leach liquor thus obtained isv recycled to ...said..alkali metal chromate solution upstream from-the pointatwhich said leadsulfate is recycled thereto.

. 6.. Themethod of claim 4 wherein the source I of the-base used to adjust the pH of the vanadate containing acid leach liquor is the alkaline leach .nquor obtained from the leaching of an alkaline chromite ore roast mass.

TOM S. PERRIN. ROBERT G. BANNER.

' REFERENCES CITED The following references are of record. in the file of this patent: 

1. IN THE METHOD OF RECOVERING VANADATES FROM A PRECIPITATE CONTAINING LEAD VANADATE BY DECOMPOSING SAID PRECIPITATE WITH SULFURIC ACID AND RECOVERING THE VANADIUM VALUES OF SUCH PRECIPITATE AS A SALT OF HEXAMETAVANADIC ACID, THE IMPROVEMENT WHICH CONSISTS ESSENTIALLY IN LEACHING SAID PRECIPITATE WITH SULFURIC ACID OF A CONCENTRATION SUBSTANTIALLY WITHIN THE RANGE OF 34.5%42.5% AND AT A TEMPERATURE SUBSTANTIALLY WITHIN THE RANGE OF 25*-70* C., TO CONVERT SAID LEAD VANADATE TO VANADIC ACID AND PRECIPITATE LEAD SULFATE, SEPARATING THE SOLUTION OF VANADIC ACID FROM THE LEAD SULFATE, ADJUSTING THE PH OF THE SOLUTION TO A POINT WITHIN THE RANGE OF 1-3 WITH A BASE SELECTED FROM THE GROUP CONSISTING OF SODIUM AND AMMONIUM BASES, AND HEATING THE SOLUTION TO A TEMPERATURE WITHIN THE RANGE OF 80* C. TO THE BOILING POINT OF THE SOLUTION TO PRECIPITATE A SALT OF HEXAMETAVANADIC ACID. 